The present invention relates to improvements in apparatus which automatically produces an alarm and/or prevents the hazardous handling of unsafe loads when overload conditions are detected by a load-responsive sensor aboard a fork-lift truck or like load-handling equipment, and, in one particular aspect, to such automatic apparatus which is substantially fail-safe and wherein associated electronic control circuitry uniquely overcomes a combination of operating defects, including: oscillations in the support of loads of near-critical level, wastefully-protracted and troublesome disabling of the load-handling equipment following occurrence of transient minor overloads, inability to sense serious overloads soon after another overloading has taken place, and false disabling of the equipment by ambient electrical interference signals.
It has been well known heretofore that excessive levels of mechanical loading of various kinds of equipment could be detected by sensors or transducers disposed at locations where they will continuously respond to at least part of the loading being experienced. One such transducer has functioned as a force pin in the clevis of a lift cylinder of a forklift truck, the pin being equipped with electrical-resistance strain gages which yield electrical signals characterizing the loads being lifted. In another arrangement, described in U.S. Pat. No. 3,570,696, a pivot pin similarly situated is used to actuate a microswitch for the signalling of lifttruck overloads. It has also been proposed, in U.S. Pat. No. 3,866,200, that excessive loading in a hoisting rig be detected via a load cell, with an output being triggered electronically when a load-characterizing signal exceeds a reference load-limit signal.
Accordingly, it is not conceptually broadly novel to sense and warn of overload conditions by electrically characterizing the loading condition of load-handling equipment and detecting when that condition exceeds a predetermined safe level. However, in practice, such a basically simple scheme is found to lack important capabilities of resolving serious problems arising out of the manner in which such load-handling equipment is actually used. In the case of a fork-lift truck attempting to handle a near-critical load, for example, the dynamics of the truck and its operation can cause the load sensor to deliver an electrical output which will oscillate with excursions above and below a preset reference level for maximum safe loading; at the same time, this involves alternate application and cut-off of load-handling power in response to the varying sensor outputs, with the result that the equipment is unstable and rendered useless and is possibly a hazard so long as the oscillations persist. If the system is then purposely made quite sluggish in operation, in an effort to avoid oscillation problems, the resulting slow responses can then admit of limited, but nevertheless dangerous, handlings of excessive loads. Further, momentary rough handling of high but safe loads can cause the system to disable the truck power, and, if one were merely to introduce a time lag to prevent such disabling on that account, then excessive loads would wrongly appear to be safe for at least a short time sufficient to admit of an accident. Moreover, ambient electromagnetic interference signals, which are prevalent in the common environments of use of industrial equipment, and including alternator noise in the case of trucks, are highly likely to cause false operations of sensitive electronic circuitry which may be used in an effort to achieve overload control. Still further, any such control system must lend itself to operations which are essentially fail-safe, and must be substantially tamper-proof in respect of efforts of impatient, or careless operators to eliminate its effects.